If you are a GitHub user, you can use the Embed GitHub Gist plugin to do so. That will give you a nicely formatted code block with many options.

If you just want to use the WordPress editor and write your code, I've had good luck with SyntaxHighlighter Evolved and WP-Syntax. You just have to pick which one you like best. Oh, and at this very moment, I'm using the Twenty Twelve Theme. I'm looking at some other responsive ones (including Frank), but haven't pulled the trigger yet.